Hot-headed males may have led to the demise of the dinosaurs in the wake of the huge asteroid impact that hit Earth 65 million years ago, say fertility experts.

A warmer climate could have led to the birth of too many male dinosaurs and not enough females, they wrote in the current issue of the journal Fertility and Sterility.

They said this would have seriously thrown dinosaur populations out of whack and could have wiped out many species.

"Crocodiles and a lot of reptiles use egg incubation temperature as a sex determining factor," said U.K. researcher and lead author Dr David Miller of the University of Leeds.

The warmer the eggs, the more males pop out, said Miller. And if the global climate was warming at the end of the Cretaceous era 65 million years ago, it could have led to too many males.

Palaeontologists and biologists had raised the idea before, said palaeontologist Dr Mark Norell of the American Museum of Natural History. But this was the first time experts in human fertility had done so.

"We thought we could bring some new angles to dinosaurs," Miller said.

It was also an opportunity to widen the horizons of fertility clinicians, Miller said, who didn't often get a chance to ponder the larger questions of how sex determination evolved and had no idea that different animals had evolved completely different approaches.

Choosing sex

In mammals, birds, all snakes, most lizards, amphibians, and some fish, sex is determined genetically, though in different ways. For example, male mammals have a Y chromosome.

Using genes instead of temperature or another environmental condition was probably nature's way of protecting mammals from sudden climate changes, Miller said.

But genes also had their pitfalls, he said. The Y chromosome could be dying out in mammals. "[The Y chromosome] doesn't recombine with eggs," said Miller. "So eventually it will just disappear."

Whatever the mechanism, sex determination is critical for the survival of species, as the right ratios of males to females must exist to successfully propagate.

Miller's team ran an analysis that showed a temperature shift could theoretically have led to a preponderance of males. Other studies have shown that when there are too few females, eventually the population dies out.

"The earth did not become so toxic that life died out 65 million years ago; the temperature just changed, and these great beasts had not evolved a genetic mechanism [like our Y chromosome] to cope with that," said Dr Sherman Silber, an infertility expert from St Luke's Hospital, St Louis, who also worked on the study.

But crocodiles and turtles had already evolved at the time of the great extinction 65 million years ago. How did they survive?

"These animals live at the intersection of aquatic and terrestrial environments, in estuarine waters and river beds, which might have afforded some protection against the more extreme effects of environmental change, hence giving them more time to adapt," the researchers wrote.

The matter of temperature and sex determination is also of concern to researchers who are watching how global warming might affect sex selection.